Add maintenance instructions for sphinx extensions/themes

[maxrjones]

Description of the desired feature

A few sphinx extensions are included in the gmt repository in doc/rst/_extensions and the sphinx_rtd_theme is included in doc/rst/themes. We should add instructions for bumping the extensions in doc/rst/source/devdocs/maintenance.rst and ideally a workflow to open an issue when new versions are released.

Relates to https://github.com/GenericMappingTools/gmt/issues/5282, https://github.com/GenericMappingTools/gmt/issues/5211.

Are you willing to help implement and maintain this feature? Yes

[PaulWessel]

I agree. For old people it is also hard to remember why when something breaks once a year...